On Tue, 24 Nov 2009, Tim Ahrens wrote: > Here in the county, all the fire repeaters have the same output > frequency & pl tone, but different input pl tones. > > They use separate radios for each area (E, W, etc). > > What they would do in our case would be to allocate (on their console) > a different radio for the additional repeater.
But dispatch is only going to have one repeater keyed up at a time, whether it's east, west, north, or south. Since dispatch has the same radio path to both repeaters, it will capture the FM receiver of both repeaters, but the repeater controller (and therefore transmitter) will not be triggered unless the proper PL tone is detected on the repeater's input. You can do this with a 16-channel Motorola Radius, some programming, and a Zetron paging unit. Once the radio is programmed that channel 1 is frequency 1 PL M1 and channel two is frequency 1 PL Z1, one pin on the back of the radio can be used to toggle between channel 1 and channel 2. In a different type of work, a tone remote would be used to send a signal over a pair of wires to the base station, where a short burst of time would tell the transmitter or receiver to change channels. These systems often sent TX1/RX1 audio on one pair, and RX2 audio on the other pair. There was only one transmitter, and often times only one receiver. I don't understand it myself, I just know it's a common thing to have done at implementation time. In much the same way, the above example of paging using a Radius and the tone remote operate on similar concepts. Either way -- you've got one radio path from dispatch on that given frequency to either repeater, so only one repeater at a time may be used. One of the local fire channels sports four repeaters on the same frequency on different sides of the county. All four of them have different PLs for receive. All four of them use the same receive PL as the transmit PL. All four of them are 100W repeaters. The only reason why there is no interference is because fires aren't fought between the repeaters. Typically, if there is an event going on both sides of the county, the 911 center uses a remote receiver or tone remote to capture the audio and send it back to central dispatch. The core concepts here are the FM capture effect, and having the stronger signal be over 10dB stronger than the other signal. -- Kris Kirby, KE4AHR Disinformation Analyst
